We don't receive certain user's emails.

Question

Users who use Microsoft365 email sometimes do not receive emails to our company.
That user is not our company.

When that user sends an email from Outlook Web App, we can receive it.
But when that user sends mail from Thunderbird, we cannot receive it.
E-mails from another person with the same domain will be received by both mailers.

What are the possible causes?

Thanks.

Answer 1

Assuming the message made it to 365:
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/message-trace-scc?view=o365-worldwide

Answer 2

If they are using Thunderbird, then the sender also needs the ability to send the messages through a SMTP server defined at the client. It could be failing there and the sender will have to tell you if they are getting a failure or bounce back message

Answer 3

Its going to be tough then. Whomever supports his messaging will need to look at the SMTP protocol logs on the SMTP server the Thunderbird client is using to see what happened to these messages.